Didemnins, antitumor and antiviral cyclic depsipeptides, were initially isolated in 1981 from the Caribbean tunicate Trididemnum sodium (Rinehart Jr., K. L., J. B. Gloer, J. C. Cook Jr., S. A. Misak, T. A. Scahill [1981] J. Am. Chem. Soc. 103:1857). So far, nine related peptides, didemnins A (3), B (4), C (5), nordidemnins A (6), and B (7), didemnins D (8), E (9), G (10), and methylene didemnin A (11) have been characterized (Rinehart Jr., K. L., J. B. Gloer, R. G. Hughes Jr., H. E. Renis, J. P. McGovren, E. B. Swynenberg, D. A. Stringfellow, S. L. Kuentzel, L. H. Li [1981] Science 212:933; Gloer, J. B. [1983] Ph. D. Dissertation, University of Illinois at Urbana-Champaign; Gutowsky, R. E. [1984] M. Sc. Thesis, University of Illinois at Urbana-Champaign). See FIGS. 1, 1A, 1B, and 1C for the structural formulas of the bold face compound numbers referred to herein. Didemnin A (3) is the simplest and most abundant constituent, and it consists of five amino acid and two non-amino acid subunits. The structure of one of the non-amino acid subunits, (3S,4R,5S)-isostatine, was originally assigned as (3S,4R)-statine, but was later found to be incorrect, and it was revised during the synthetic study of 3 (Rinehart, K. L., V. Kishore, S. Nagarajan, R. J. Lake, J. B. Gloer, F. A. Bozich, K.-M. Li, R. E. Maleczka Jr., W. L. Todsen, M. H. G. Munro, D. W. Sullins, R. Sakai [1987] J. Am. Chem. Soc. 109:6846; Rinehart, K. L., V. Kishore, K. C. Bible, R. Sakai, D. W. Sullins, K.-M. Li [1988] J. Nat. Prod. 51:1; Sakai, R., First Year Paper I, University of Illinois at Urbana-Champaign). Of the didemnins isolated so far, 4 has shown the most potent biological activities, and strong antitumor efficacy has led this compound into Phase II clinical trials. Structurally, all didemnins except for nor- and methylene didemnins contain 3 as the basic skeleton, and the only differences between them are their side chains. Differences in the side chain or simple modifications in the ring functional groups, however, cause drastic changes in their biological properties (Gloer [1983], supra). These interesting structure-activity relationships stimulated us to find new didemnins for further chemical and biological studies. See U.S. Pat. No. 4,548,814, which discloses and claims processes for preparing didemnins A, B, and C, and nordidemnins A, B, and C. Also, see U.S. Pat. No. 4,493,796, which discloses and claims didemnins A, B, C, D, and E.